1. Field of the Invention
The present invention relates to zinc oxide-based ceramic compositions intended particularly for low and medium voltage varistors.
2. Discussion of the Background
A varistor is an electric component in which the resistance decreases strongly when the voltage, applied to its terminals increases. This remarkable characteristic allows this component to be used in applications for protection against overvoltages in electric circuits.
Several types of resistors are known which vary in a non-linear manner as a function of the voltage applied to their terminals, including Zener diodes which are used in the lowering of low voltages, and varistors based on zinc oxide or other metallic oxides manufactured by ceramic techniques. These components are low in cost and can be used over a large voltage range (practically from 3 volts to a few kilovolts).
Varistors have been widely used to stabilize the voltage of electric circuits or to remove the overvoltages induced in a circuit.
The electrical characteristics of such varistors is expressed by the ratio: EQU I=K.times.V.sup..alpha.
where V is the voltage at the terminals of the varistor, I the current which passes through it, K a constant depending on the geometry of the varistor element and on the technology used to make it and .alpha. the coefficient of non-linearity. The varistor effect is more pronounced when the value of the coefficient is high.
To characterize the non-linearity of the varistor, the ratios V.sub.1mA /V.sub.10.mu.A and V.sub.10A /V.sub.1mA are frequently used, that is, the ratios of the voltages corresponding to 1mA, 10.mu.A and 10A, 1mA respectively. V.sub.1mA /V.sub.10.mu.A characterizes the escape current of the varistor and V.sub.10A /V.sub.1mA the lowering. The ideal and theoretical case is where these values are equal to 1. In practice, these ratios are greater than 1.
Varistors based on zinc oxide and containing additives have already been described, for example in EP 29 749. The compositions cited in EP 29 749 contain as additives one or several combinations of bismuth, cobalt, manganese, boron, aluminum, silver, gallium, antimony, chromium, nickel and silicon oxides. These compositions generally have a good coefficient of non-linearity but do not simultaneously provide:
a good current-voltage characteristics,
good resistance to current shocks, and
high stability of the electrical properties in moist heat, even in the presence of a coating.